One of liberalism's many problems is that once an idea or program is proved wrong and unworkable, liberals rarely acknowledge their mistake and examine the root cause of their error so they don't repeat it.

Cameron said in Britain different cultures have been encouraged to live separate lives: "We have failed to provide a vision of society to which they feel they want to belong." Here I would take issue with an otherwise excellent speech. It isn't that Britain has failed to provide such a society. Rather, many of those coming to Britain (and increasingly France, Germany and the U.S.) don't want to become a part of those cultures, which they regard as corrupt and anti-God.

Britain's policy should be to require -- yes, require -- immigrants to become part of a melting pot and not individual vegetables floating around in a multicultural stew. Otherwise, they should not be admitted.

When critics of multiculturalism and unbridled immigration warned of the inevitability of a loss of nationhood and national identity, they were denounced as alarmists, even racists.

The late British parliamentarian Enoch Powell suffered such attacks (and earned many kudos) when he repeatedly warned about the dangers of open-ended immigration without assimilation. In a controversial speech to a Conservative Party conference in 1968, Powell began his address, known as "Rivers of Blood," with what ought to be an obvious statement: "The supreme function of statesmanship is to provide against preventable evils. In seeking to do so, it encounters obstacles, which are deeply rooted in human nature."

Powell argued that when it comes to multiculturalism and immigration, Britain had failed in that mandate. Looking into the future, Powell accurately predicted what has come to pass from mass and uncontrolled immigration: "Of course, it will not be evenly distributed from Margate to Aberystwyth and from Penzance to Aberdeen. Whole areas, towns and parts of towns across England will be occupied by sections of the immigrant and immigrant-descended population."

Powell wasn't so much railing against immigrants, though his critics read it in those terms, but against Britain's refusal to integrate them into British culture.

And then Powell let the timid class have it with this line: "There could be no grosser misconception of the realities than is entertained by those who vociferously demand legislation as they call it 'against discrimination', whether they be leader-writers of the same kidney and sometimes on the same newspapers which year after year in the 1930s tried to blind this country to the rising peril which confronted it, or archbishops who live in palaces, faring delicately with the bedclothes pulled right up over their heads. They have got it exactly and diametrically wrong."

In 1968, Britain still had time to reverse course, but because its leaders didn't want to be called "racists" and immigrants were doing jobs British citizens were increasingly reluctant to do (sound familiar?) the floodgates were left open. It may be too late for Britain, as it may be too late for France and Germany.

It isn't too late for the United States, though it is getting close. Too many American leaders suffer from the same weak-kneed syndrome that has gripped Britain. Who will tell immigrants to America that the days of multiculturalism are over and if they want to come to America, they must do so legally and expect to become Americans with no hyphens, no allegiance to another country, and no agenda other than the improvement of the United States?

Enoch Powell was right four decades ago. David Cameron is right today. If British leaders had listened to Powell then, Cameron would not have needed to make his Munich speech.